The invention involves novel compounds and compositions, which can be useful as extractants for the recovery of uranium from wet-process phosphoric acid (hereinafter, sometimes, "WPPA"), especially WPPA prepared by a hemi-hydrate route.
These compositions are multidentate ligands which are particularly suited for the formation of organic-soluble complexes of tetravalent uranium, that is U(IV) or U.sup.+4, and in so doing can accomplish transfer of this ion from strong phosphoric acid solutions to organic solutions from which the U(IV) can conveniently be recovered.
The compounds can be expressed by the following formulas 1 and 2: ##STR2## wherein R.sup.1 and R.sup.2 are the same or different and are selected from alkyl, alkoxyl, alkylaryl and alkylaryloxyl groups having from 1 to about 18 carbon atoms or hydroxyl;
R.sup.3 is independently selected from substituted or unsubstituted alkyl or alkylaryl groups having 1 to about 18 carbon atoms or hydrogen; PA1 R.sup.4 is independently selected from alkyl or alkylaryl groups having 1 to about 18 carbon atoms; provided that the sum of carbon atoms of the R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups is at least 15; and PA1 R.sup.5 is independently selected from substituted and unsubstituted alkyl or alkylaryl groups having 1 to about 18 carbon atoms or a polymeric group; provided that the sum of the carbon atoms of the R.sup.1, R.sup.2 and R.sup.5 groups is at least 16. PA1 R.sup.3 is independently selected from substituted and unsubstituted alkyl and alkylaryl groups having 1 to about 18 carbon atoms or hydrogen; PA1 R.sup.4 is independently selected from alkyl and alkylaryl groups having 1 to about 18 carbon atoms; provided that the sum of the carbon atoms of the R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups is at least 15; PA1 R.sup.5 is independently selected from substituted and unsubstituted alkyl and alkylaryl groups having 1 o about 18 carbon atoms or a polymeric group; provided that the sum of the carbon atoms of the R.sup.1, R.sup.2 and R.sup.5 groups is at least 16.
Substituted alkyl and alkylaryl groups include alkyl and alkylaryl groups substituted with moieties, such as fluoro, chloro, bromo, iodo and hydroxyl groups, that are not deleteriously attacked during preparation of the compounds of formulas 1 and 2, and, preferably, not attacked by constituents when the compounds are used as extractants.
Useful for extracting U(IV) are compounds of formula 2 wherein R.sup.5 is a polymeric group.
The novel compounds of this invention can be prepared by a method similar to that described by C. H. Roy in U.S. Pat. No. 3,422,021 in which 2-phenylethylene-1,1-diphosphonic acid was described along with substituted methylenediphosphonic acids as having utility as detergent builders. The compounds disclosed in U.S. Pat. No. 3,422,021 are not useful for the extraction of U(IV) from aqueous phosphoric acid.
Compositions similar to compounds of the general formulas 1 and 2 have been reported. For example, J. D. Spivak (U.S. Pat. Nos. 3,714,300 and 3,776,844) claims novel esters of general formula 3, which have utility as hindered phenol oxidation stabilizers. ##STR3## Similarly, the compound of formula 4 above is reported (Jpn. Kokai Tokkyo Koho 8,098,193) to have utility as a herbicide. Compounds of formulas 3 or 4 are not useful for extracting U(IV) from aqueous phosphoric acid.
Solvent extraction technology exists for the commercial recovery of uranium from conventional dihydrate phosphoric acid which has a concentration of about 28-30% P.sub.2 O.sub.5. Thus, a mixture of di(2-ethyl-1-hexyl)phosphoric acid, hereinafter, sometimes, "DEHPA" and tri(n-octyl)phosphine oxide, hereinafter, sometimes, "TOPO," in a molar ratio of about 4 to 1, is used as the extractants in a 2-cycle process used to extract uranium as U(IV) (F. J. Hurst, et al., Ind. Eng. Chem, Process Des. Dev., 11(1), 122-8 (1972)). These extractants have the formulas: ##STR4##
Similarly, the mixture (commonly referred to as "pyroesters") produced by the reaction of alcohols of 8-10 carbon atoms with phosphorus pentoxide in a 2 to 1 molar ratio is used as the extractant in a one-cycle process for the extraction of U(IV) from dihydrate acid (D. A. Ellis, The Recovery of Uranium from Industrial Phosphoric Acids by Solvent Extraction, Report No. Dow-81, Dow Chem. Co. (1952)).
A mixture (commonly referred to as "OPAP") of nearly equimolar ratio of mono(4-t-octylphenyl)phosphoric acid (referred to as "MOPPA") and di(4-t-octylphenyl)phosphoric acid (referred to as "DOPPA") of formulas: ##STR5## is used as the extractant in the first cycle of a two-cycle process to extract uranium as U(IV) from WPPA (F. J. Hurst and D. J. Crouse, Ind. Eng. Chem. Process Des., Dev., 13(3), 287-91 (1974). The DEHPA-TOPO mixture described above is used as the extractant in the second cycle of this process to extract uranium as U(VI).
Pyrophosphate organic esters have high extraction coefficients for the removal of U(IV) from WPPA. However, pyrophosphate esters are not hydrolytically stable in WPPA. The compounds of formulas 1 and 2 have higher extraction coefficients for removal of U(IV) from WPPA and are hydrolytically stable in WPPA.
The difficulty of recovering uranium from more concentrated phosphoric acid solutions, such as the product of a hemihydrate WPPA process, has been described (J. J. Kohler, et al., Recovery of Uranium from Hemihydrate Process Phosphoric Acid (40 to 50% P.sub.2 O.sub.5) by Solvent Extraction, paper presented at 180th Annular ACS Meeting Las Vegas, August, 1980).
Uranium is not presently recovered commercially from WPPA produced by a hemihydrate process (referred to herein as "hemihydrate WPPA"). Such acid is considerably more concentrated (36-50% P.sub.2 O.sub.5) than WPPA produced by a conventional dihydrate process (26-31% P.sub.2 O.sub.5).
While uranium can be recovered by any of the above referenced processes from dihydrate WPPA, uranium is difficult to recover from hemihydrate WPPA. The extractants used in these processes have prohibitively poor extractant strength for use with the more concentrated WPPA (e.g., hemihydrate WPPA), or have precipitation, solubility, or fouling problems when used to extract more concentrated WPPA, or are hydrolytically unstable and are degraded and lost during extraction in more concentrated WPPA. The novel compounds of the present invention present a great improvement over such prior art extractants, particularly for use with more concentrated WPPA, such as, hemihydrate WPPA.
Additional information regarding uranium recovery processes is set forth in: Phosphorus & Potassium (99), 31-3, (1979); A. P. Kouloheris, Chem. Eng., Aug. 11, 1980, pp. 82-4; B. F. Greek, O. W. Allen and D. E. Tynan, Ind. Eng. Chem., 49 (4), 628-38 (1957).